In recent years, highly efficient transmission through digitization is achieved in radio communication, such as wideband code division multiple access (W-CDMA). When a scheme of multi-level phase modulation is applied in radio communication, a technique is adopted by which the amplification characteristics of a transmission-use electronic amplifier at the transmission-side are linearized to suppress nonlinear distortion and reduce electrical power leakage to adjacent channels. If electrical power efficiency is improved using an amplifier inferior in linearity, a technique for compensating nonlinear distortion of signals is adopted (see, e.g., Japanese laid-Open Patent Publication No. 2006-253749 and International Publication Pamphlet No. 2007/036990).
The conventional techniques, however, pose a problem of difficulty in accurately compensating signal distortions. For example, in mobile communication, such as W-CDMA, a transmitting apparatus uses great electrical power for transmission, which causes significant nonlinear distortion of the transmission signal, raising a side lobe of the frequency spectrum of the transmission signal and thereby, leading to a leak of electrical power of the transmission signal to adjacent channels. Such electrical power leakage creates noise, which deteriorates the communication quality of adjacent channels. If a wideband signal is used as a transmission signal, in particular, a memory effect becomes larger than in the case of using a narrowband signal. A larger memory effect results in difficulty in compensating distortion near the transmission signal caused by the memory effect.